The present invention relates to the preparation of particulate cellulose and more specifically to the enzymatic preparation of cellulose powder for tablet making.
Cellulose powder, such as microcrystalline cellulose, has a wide variety of commercial applications, as for example, in food, pharmaceutical and cosmetic preparations. In particular, cellulose powder is especially useful in forming tablets to serve as carriers for pharmaceuticals, essential oils and the like, which are easily absorbed on the extensive internal surface of the tablet. Such tablets are relatively insensitive to water vapor, being free from amorphous cellulose, but break up quickly upon contacting water, releasing the active ingredient.
Commercial cellulose powder is made by subjecting wood pulp containing amorphous and crystalline forms of cellulose to acid hydrolysis at elevated temperature, on the order of 105.degree. C. or greater, for about 15 minutes to one hour, followed by mechanical disintegration of the cellulose hydrolysate. The process is generally carried out by hydrolyzing a pure grade of wood pulp with hydrochloric acid in order to prevent the formation of undesirable by-products from secondary reactions. The hydrolysis reaction removes amorphous cellulose material leaving a substantially insoluble residue comprising microcrystals of cellulose, which is commonly referred to in the art as "level-off degree of polymerization" (LODP) cellulose. Detailed information regarding the production of LODP cellulose and products derived therefrom are provided in U.S. Pat. No. 2,978,446 to Battisa et al.
The LODP value is dependent primarily upon the starting cellulosic material and to a lesser extent upon the severity of the hydrolyzing conditions. In general, the LODP of native cellulose fibers is in the range of between about 200 and 300, whereas that derived from regenerated cellulose lies in the range of from 25 to about 60.
It has been the general view of those knowledgeable about the manufacture and applications of cellulose powder that the LODP value for a given cellulosic material had to be achieved in order to obtain a commercially useful product, and that the only way in which to arrive at the LODP value was by acid hydrolysis, as described above. This view was based primarly on the belief that only inorganic acids had the requisite reactivity and mobility to attack the less accessible portions of the cellulose structure, and produce LODP cellulose.
In the prior art, although other methods of cellulose degradation, e.g. enzymatic hydrolysis have been studied and reported on, such methods have not been considered appropriate for commercial cellulose powder production. For the most part, prior studies of enzymatic hydrolysis of cellulosic materials have been undertaken for the purpose of learning how cellulose enzymes act on raw cellulosic materials, with a view toward using enzymatic hydrolysis as a means of converting cellulose into a soluble sugar solution, and thereby provide a practical process of cellulose utilization. Generally, the aim of such prior studies has been to effect complete hydrolysis of the cellulosic starting material, and not to effect enzymatic hydrolysis of cellulose for the preparation of commercially useful cellulose powders.